Shahrestani, Shahed, Sanislav, Ioan V., Liu, Yang, Blake, Kevin, Kumar, Avish, and Askew, Shane (2026) Determination of ferrous iron in biotite by EPMA using the flank method: development of a mineral-specific calibration. Chemical Geology, 726. 123737.

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Abstract

The oxidation state of iron in biotite provides key information on redox conditions, magma evolution, and fluid–rock interactions in igneous, metamorphic, and hydrothermal systems, yet routine determination of Fe2+ and Fe3+ by electron probe microanalysis (EPMA) remains difficult. This study presents a biotite-specific calibration of the flank method for estimating Fe2+ from Fe Lβ/Lα intensity ratios. Rather than defining flank positions from garnet end-members, as in the conventional approach, flank positions were established directly from biotite spectra, exploiting the comparatively narrow Fe3+/ΣFe range of biotite to achieve more consistent spectral behaviour. Twelve biotite samples spanning a wide range of total iron content (8.29–22.61 wt% ΣFeO) were analysed by EPMA and independently characterised by wet chemistry to calibrate and test the method. A simple linear regression of Fe2+ against Lβ/Lα produced a strong correlation with wet-chemistry values (R2 = 0.95, average error 0.47 wt%), and incorporating total iron (ΣFe) into a multiple regression model improved the prediction of Fe2+/ΣFe, accounting for matrix-dependent effects associated with total Fe concentration. Leave-one-out cross-validation was applied to evaluate the performance of both models, and independent validation against a well-characterised reference biotite (LP-6) yielded Fe2+ estimates within 0.4–0.6 wt% of the reported composition. These results demonstrate that mineral-specific calibration of the flank method, combined with controlled instrumental conditions, offers a practical, and in-situ alternative to Mossbauer spectroscopy or synchrotron XANES for constraining Fe2+/Fe3+ in biotite, with potential application to other Fe-bearing silicates and to broader petrogenetic, ore-deposit, and metamorphic studies in which iron oxidation state provides information on mineral-forming conditions.

Item ID:	94077
Item Type:	Article (Research - C1)
ISSN:	1872-6836
Copyright Information:	© 2026 Published by Elsevier B.V.
Date Deposited:	30 Sep 2026 01:47
FoR Codes:	37 EARTH SCIENCES > 3703 Geochemistry > 370302 Inorganic geochemistry @ 70% 37 EARTH SCIENCES > 3705 Geology > 370505 Mineralogy and crystallography @ 30%
SEO Codes:	28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 100%
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